1. Field of the Invention
The present invention relates to a thermal displacement correction apparatus that is equipped in a machine tool and corrects a thermal displacement amount arising in the machine tool.
2. Description of the Related Art
In a machine tool, since feed screws and a main spindle are driven by motors, the main spindle and the feed screws expand to cause their displacements in machine positions due to heat generated from the motors, frictional heat caused by rotation of bearings, and frictional heat in engagement parts of ball screws and ball nuts of the feed screws. That is, a displacement arises in a relative positional relation between a workpiece and a tool which are to be positioned. This variation in machine positions due to the heat is problematic when performing high-precision processing.
As a method for removing this displacement in machine position due to the heat, there have been conventionally adopted methods of providing a cooling apparatus, configuring a structure in which the ball screw of the feed screw is given initial tension so as not to be affected by the expansion due to the heat, and providing a displacement sensor or a temperature sensor and correcting a command position on the basis of a detected displacement or a detected temperature.
In correction where a thermal displacement amount is calculated from an operation of the machine tool, temperatures of respective parts of the machine tool, and the like, there can be a case where the thermal displacement amount cannot be correctly calculated, which causes a large difference (correction error) between a thermal displacement correction amount and an actual thermal displacement amount and leads to incorrect correction. In such a case, the thermal displacement correction amount is typically increased or decreased for adjustment to improve correction precision.
Prior art technique regarding adjustment of thermal displacement correction amount is disclosed, for example in Japanese Patent Application Laid-Open No. 11-90779, in which an actual displacement amount is measured using a measurement device when performing machining, and the actual displacement amount measured is compared with the thermal displacement correction amount to obtain an adjustment value, and the adjustment value thus obtained is added to, or subtracted from, the thermal displacement correction amount which is calculated, thereby changing and adjusting the thermal displacement correction amount.
The aforementioned adjustment of the thermal displacement correction amount disclosed in Japanese Patent Application Laid-Open No. 11-90779 requires measurement of the actual displacement amount at a machining point when adjusting the thermal displacement correction amount, which is burdensome. Moreover, although this technology provides a corresponding relation between the maximum displacement amount and the average movement distance, in some cases, the corresponding relation may take a large deviation depending on a machine property or machining environment.
In the conventional method, an adjustment value for adjusting thermal displacement correction value is constant value after it is set in advance or an actual displacement amount is measured. After that, in some cases, the adjustment value has to be changed depending on an external environment or a machining situation. In this case, it is sometimes necessary to carry out a plurality of times of manual setting or measurement with machining cycle suspended, for matching the thermal displacement correction amount to the actual displacement amount.
FIG. 8 is a diagram for explaining the conventional art.
“Corresponding Relevant Data between Thermal Displacement Amount and Main Spindle Movement” in (1) shows that corresponding relevant data between the thermal displacement amount and the rotation movement amount of main spindle in the machine is obtained by preliminarily performing a thermal displacement experiment, and the data thus obtained is stored in the machine (more specifically, stored in a memory of a numerical controller that controls the machine). With the conventional art which is disclosed in Japanese Patent Application Laid-Open No. 11-90779 above, however, the relevant data regarding the thermal displacement experiment cannot be changed by a user of the machine, and furthermore, the data is applicable only to the specified machine and main spindle.
“actual measurement” in (2) shows that, when there is a difference between the correction value (obtained by multiplying the thermal displacement correction amount by the adjustment value) and the actual displacement value which exceeds a predetermined level, it is necessary to carry out a plurality of times of actual measurement to change the adjustment value for correcting the correction value.
FIG. 9 is a diagram for explaining that, for changing the adjustment value depending on the external environment or machining situation in the conventional art, it is necessary to suspend the machining cycle to carry out measurement.
FIG. 9 illustrates two times of measurement performed at the time points of measurement 1 and measurement 2. At the time point of measurement 1, since there is a large difference between the thermal displacement correction amount and the actual displacement amount, the adjustment value is changed. Likewise, at the time point of measurement 2, since there is a large difference between the thermal displacement correction amount and the actual displacement amount, the adjustment value is changed again. In the conventional art, the adjustment value is set to a constant numerical value after the measurement. Hence, for improving precision in correcting the thermal displacement amount, it is necessary to carry out measurement again to change an adjustment amount for adjusting the thermal displacement amount to an appropriate value, which problematically causes the machining cycle to be suspended for measurement.